Numerous types of electronic projection apparatus have been proposed for projecting digital images. Currently, projection apparatus of this type serve primarily for business presentations and other types of projection apparatus. However, in the future, electronic projection apparatus are expected to compete with conventional film-based projectors, such as in movie theaters. Electronic projectors are also likely candidates for use in the front-projection home theater market. Among the more promising solutions for electronic imaging, laser projection offers a number of advantages over other light sources, with benefits including wide color gamut and high efficiencies, for example.
Linear SLMs, which could also be considered as one-dimensional spatial light modulators, have some advantages over two-dimensional LCD and DMD area spatial light modulators that have typically used for business projection systems. Inherent performance advantages for linear modulator arrays include the capability for higher resolution at reduced cost. In addition, linear arrays are more suitable modulators for laser light than are their two-dimensional LCD and DMD counterparts. Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360 (Bloom et al.) are one earlier type of linear modulator array that offers a workable solution for high-brightness imaging using laser sources, for example.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in commonly assigned U.S. Pat. No. 6,307,663, entitled “Spatial Light Modulator With Conformal Grating Device” issued Oct. 23, 2001. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device, with GEMS standing for Grating ElectroMechanical System. The conformal GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of conformal GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system, providing a high degree of system flexibility and allowing the use of lower cost optics. When used with laser sources, GEMS devices provide excellent brightness, speed, and contrast.
Commonly assigned U.S. Pat. Nos. 6,411,425 and 6,476,848 (both to Kowarz et al.) disclose imaging systems employing GEMS devices in a number of printing and display embodiments. As with its GLV counterpart, a GEMS device modulates a single color and a single line of an image at a time. Thus, forming a color image using GLV or GEMS devices requires suitable techniques either for sequencing illumination and modulation data for each color to a single linear modulator or for combining separately modulated color images. Other types of modulators, such as LCD and Digital Micromirror Device (DMD) area spatial light modulators, could be used for forming color images by modulating a laser light source; however, inherent cost, performance, and/or color quality drawbacks with these devices make them less desirable candidates for laser-based projection apparatus.
Although laser light offers advantages such as good color gamut and long component lifetimes, potential hazards associated with scanned coherent laser light are a concern that hinders development efforts to provide commercial quality laser projection. Projection of laser light onto a diffuse screen effectively scatters the coherent beam, so that the resulting displayed image may be enjoyed without safety concerns. However, a lingering area of concern for employing laser projectors relates to inadvertent, accidental exposure to laser illumination caused when an observer, located within the projected laser beam's path, looks back into the projector. With conventional light-based projectors, a person inadvertently gazing back into the projector would certainly find it uncomfortable to maintain her gaze, but would not risk serious eye damage. With laser-based projectors, however, a viewer can be at higher risk of eye damage, unless some form of protection is provided. Lasers proposed for use in projection apparatus may have output power in the range of 0.5 to 20 watts, or even higher.
Interlocks are one conventional method for disabling laser illumination when a protective cover is removed or when some other event indicates that built-in safety features for a laser device have been compromised. In order to implement interlock protection, some type of sensor must be activated or a hard-wired connection must be interrupted. However, conventional interlock solutions would not be well suited for disabling laser-based projection when an audience member or some type of object is interposed in the path of projected light.
Laser light show systems in current use employ a safety feature known as velocity threshold interlock. Rapidly scanning beams dramatically reduce the exposure level of the scanning spot compared non-scanned or slowly scanning beams. The laser light show projector systems monitor the angular velocities of their scanner sub-systems and blank the beams when velocities fall below a predetermined threshold. The feature guards against the possibility of high projected laser irradiance which would occur if, for example, the scanner used in such systems failed to scan the beam.
However, the aforementioned method of blanking the entire display screen is particularly annoying for many in the audience. Thus, it can be seen that there is a need for a digital projection apparatus that takes advantage of laser brightness, but minimizes imposing annoyance upon the viewing audience, should an obstruction occur within the laser beam's path.